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10.1146/annurev.arplant.55.031903.141729
Annu. Rev. Plant Biol. 2004. 55:225–61
doi: 10.1146/annurev.arplant.55.031903.141729
c 2004 by Annual Reviews. All rights reserved
Copyright First published online as a Review in Advance on January 7, 2004
PHYTOESTROGENS
Richard A. Dixon
Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401;
Centre for Novel Agricultural Products, University of York, Heslington Road, York,
United Kingdom; email: [email protected]
Key Words isoflavone, lignan, soybean, flaxseed, nutraceutical
■ Abstract Collectively, plants contain several different families of natural products among which are compounds with weak estrogenic or antiestrogenic activity
toward mammals. These compounds, termed phytoestrogens, include certain isoflavonoids, flavonoids, stilbenes, and lignans. The best-studied dietary phytoestrogens
are the soy isoflavones and the flaxseed lignans. Their perceived health beneficial properties extend beyond hormone-dependent breast and prostate cancers and osteoporosis
to include cognitive function, cardiovascular disease, immunity and inflammation, and
reproduction and fertility. In the future, metabolic engineering of plants could generate novel and exquisitely controlled dietary sources with which to better assess the
potential health beneficial effects of phytoestrogens.
CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ISOFLAVONOID PHYTOESTROGENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Occurrence and Dietary Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bioavailability and Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential Health Beneficial Effects of
Isoflavonoid Phytoestrogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metabolic Engineering of Isoflavone Phytoestrogens . . . . . . . . . . . . . . . . . . . . . . . .
Phytoestrogens and Sex Determination in Plants . . . . . . . . . . . . . . . . . . . . . . . . . . .
FLAVONOID PHYTOESTROGENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential Health Beneficial Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIGNAN PHYTOESTROGENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential Health Beneficial Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OTHER CLASSES OF PHYTOESTROGENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REASSESSING THE HEALTH BENEFICIAL EFFECTS
OF PHYTOESTROGENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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INTRODUCTION
In addition to being a source of compounds necessary for human nutrition, certain
plant foods also contain compounds that may have long-term effects on human
and animal health. Among the most important are the phytoestrogens. The term
phytoestrogen first appeared in the literature in the late 1980s, and phytoestrogens
have since become a major topic of research, as shown by the numbers of citations
including the term listed in biological abstracts. The numbers parallel the use of
the term nutraceutical (Figure 1). Most of the recent citations on phytoestrogens
address potential health promotion in mammalian systems. Two of the most important and potent phytoestrogens, equol and enterolactone, do not accumulate in
plants but are microbial degradation products of the soy isoflavone daidzein and
the flaxseed lignans secoisolariciresinol and matairesinol, respectively, formed in
the gut (165, 191). As shown in Figure 1, the potential estrogenic effects of lignans
were realized some time later than those of isoflavones, and thus a significantly
larger percentage of the research reports on lignans are nonhealth related compared
with those on isoflavones.
Estrogenic compounds appear in more than 300 plant species, but few of these
are consumed by animals or humans. Some plants contain steroidal estrogens,
Figure 1 Citation statistics for papers relating to phytoestrogens. Graph shows the
number of citations per year from 1990 to 2002, with various terms appearing in
the titles or abstracts, or as key words. The numbers were obtained from searching
biological abstracts. The symbols on the graph represent annual citations for the terms
genistein ( e), phytoestrogen (¤), lignan (¨), nutraceutical (N), and phytoestrogen plus
lignan (•).
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but as these are essentially based on the same structures that occur naturally in
animals, they are not considered phytoestrogens by the strictest definition (3). The
common phytosterols such as β-sitosterol, campesterol, and stigmasterol do not
bind to human estrogen receptors (ER) and do not exert estrogenicity in female rats
(15). Therefore, we do not further discuss plant sterols here. This leaves the soy
isoflavones genistein and daidzein, the chickpea isoflavone biochanin A, the clover
isoflavone formononetin, the isoflavonoid-derived coumestan coumestrol, and the
flaxseed lignans as the major phytoestrogens relevant for human and animal health
(Figure 2, see color insert; Figure 3). Almost all these compounds occur in the plant
in glycosylated forms, and importantly, the bioavailability of the glycoconjugates
might be different from that of the unsubstituted aglycones, which are often used
in animal feeding and model cell culture studies.
There are several excellent reviews documenting and evaluating the large and
complex body of evidence purporting to demonstrate the health beneficial effects
of phytoestrogens for humans and laboratory animals (2–4, 7, 166). This review
departs from the main focus of these articles by describing recent advances in our
understanding of the potential health benefits of dietary phytoestrogens and by
including the biochemistry of phytoestrogens in the plant.
ISOFLAVONOID PHYTOESTROGENS
Occurrence and Dietary Sources
The isoflavonoids enjoy a restricted distribution in the plant kingdom, being mostly
limited to the subfamily Papilionoideae of the Leguminosae (44). The first evidence that plants contained estrogenic compounds came from investigations on the
negative effects of clovers on fertility in cattle and sheep, reviewed in (163). Breeding to reduce the isoflavone phytoestrogen formononetin (Figure 2B) became an
important goal, and new high-yielding clover varieties with reduced formononetin
levels were recently released (23). Isoflavone phytoestrogens may also affect avian
fertility. In 1976 it was hypothesized that California quail switched their feeding
preference to legumes containing formononetin to control (reduce) their fertility
at times of food shortage (114).
Isoflavonoids are derived directly from flavanones, compounds ubiquitously
present in plants. They differ from the flavonoids in having the B-ring linked to
the 3- rather than the 2-position of the central heterocycle (see Figure 2B). The
overall structural variation among isoflavonoids is surprisingly large, involving not
only the number and complexity of substituents, but also different oxidation levels
of the heterocycle and even the presence of additional heterocyclic rings, usually
methylenedioxy (formed by cyclization between vicinal hydroxyl and methoxyl
groups) or dimethylchromene (formed by cyclization between vicinal hydroxyl
and monoprenyl groups) (44). Those isoflavonoids that are dietary phytoestrogens generally fall into the class of simple isoflavones and their glycosides although coumestrol (Figure 2A), a more complex isoflavonoid derivative, is also a
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phytoestrogen. The number of known isoflavone glycosides [e.g., genistin (genistein 7-O-β-D-glucopyranoside)], is small when compared with the vast range of
known flavonoid glycosides, and O-glycosides predominate.
Natural sources from which isoflavonoids and their glycosides have been isolated are listed in an excellent review by Dewick (44). However, relatively few
species provide dietary isoflavones to humans. These include soybean (the seeds
of which accumulate high levels—milligram amounts—of daidzein and genistein
and their glycosides plus smaller amounts of free and conjugated glycitein and
coumestrol), chickpea (biochanin A), and alfalfa sprouts (formononetin glycosides
and coumestrol) (58, 156, 190). Figure 2 shows the structures of these compounds,
along with that of the human estrogen 17β-estradiol. Note the similarity in structure between estradiol and isoflavone (Figure 2A), with the distance between the
7- and 40 -hydroxyl groups on isoflavones almost identical to that between the C3
and C17 hydroxyls of estradiol. O-methylation of isoflavones decreases their estrogenicity as determined by in vitro ER-binding assay, with formononetin and
biochanin A less potent than daidzein and genistein, respectively. Isoprenylation
of methylated isoflavones may increase estrogenic activity (160), although prenylated isoflavonoids are not common or quantitatively important components of the
human diet.
Coumestrol has higher binding affinity for ER than has genistein, and exhibits
strong estrogenic activity, similar to that of estradiol, in the rat uterotrophic assay (180). Coumestrol can suppress estrous cycles when fed to female rats, and
negatively affects the sexual behavior of male offspring (193), as well as having
potential mutagenic effects (50). Coumestrol also has metabolic effects that are
independent of its estrogenicity, including increasing lipid synthesis and glycogen
catabolism in perfused rat liver (145). Because the main sources of coumestrol are
alfalfa and clover, these potentially deleterious effects are probably more relevant
for veterinary than for human medicine, although clover is used as a medicinal
plant (143).
One gram of powdered soybean chips contains nearly 800 µg of daidzein and
over 500 µg of genistein (primarily as glycosides), but one gram of soy protein still
retains high levels of approximately 150 µg of daidzein and 250 µg of genistein.
Highly processed soy products such as miso and soy sauce contain much lower to
negligible levels of genistein than does tofu, a major source of isoflavones in the
Asian diet. In humans eating a soy-rich diet, ingested isoflavone levels can be very
high, as determined by urinary excretion (108).
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
Figure 3 Biosynthesis and metabolic conversions of the major dietary lignans. The
reactions in gray shading represent microbial metabolism of plant lignans to the “mammalian lignans.” The enzymes are PLR, pinoresinol/lariciresinol reductase and SDH,
secoisolariciresinol dehydrogenase. Glucosyltransferase(s) and β-glucosidases catalyze the interconversions between secoisolariciriesinol and its diglucoside. UDPG,
uridine diphosphate glucose.
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A full understanding of dietary levels of phytoestrogens is essential for epidemiological studies in which phytoestrogen intake is usually estimated from
food frequency questionnaires. Therefore, much effort has gone into developing
methods for the accurate quantification of phytoestrogens in plants, plant extracts,
and botanicals, as well as in body tissues and fluids. The methods range from
those that target specific compounds, such as high-performance liquid chromatography (HPLC) coupled to UV spectroscopy and/or mass spectrometry (17, 96)
and immunological methods such as enzyme-linked immunosorbent assay (19), to
methods that do not chemically identify the active compound(s), such as in vitro
estrogencity tests (the rodent uterotrophic assay) (47), microtitration assays with
human ER (65), or analysis of ER binding using a reporter gene assay in yeast
(134). Generally speaking, HPLC with UV detection is one of the simplest methods for determination of isoflavone phytoestrogen levels, both in plant extracts and
animal tissues or body fluids (119, 189). However, the sensitivity of this method
has been questioned, and isotope dilution gas chromatography mass spectrometry
with selected ion monitoring was developed as a significantly improved method
for accurate quantitation (129). The method involves inclusion of deuterated internal standards at the initial extraction stage, and has resulted in an expanded
view of those plant sources that contain isoflavones, albeit at low levels (129) (see
Table 1).
Several databases and compendia have been developed to complement food frequency questionnaires, based on quantitative analysis of isoflavones, coumestans,
and lignans by the methods outlined above (78, 150, 156). In one such study, these
compounds were determined in each of 112 food items/groups. As expected, high
levels of genistein and daidzein, as well as substantial amounts of coumestrol, were
found in traditional soy-based foods, as well as soy protein isolate, soy concentrate,
or soy flour added to foods. Various types of sprouts and dried fruits, garbanzo
beans, asparagus, garlic, and licorice, were also substantial contributors of dietary
phytoestrogens (78, 150). Table 1 provides a summary of isoflavonoid phytoestrogen levels in some of the food sources analyzed by Adlercreutz and coworkers
(129). All the species reported to contain significant levels of isoflavones are members of the Leguminosae.
The placenta does not appear to be a barrier to daidzein and related phytoestrogens (41), so mothers with high dietary isoflavone intake could provide the fetus
with significant exposure to the compounds. There has also been considerable interest and concern about the effects of soy phytoestrogens in infant formula. The
concern is based on results of some animal studies which, for example, have shown
that neonatal exposure of mice to 50 mg genistein/kg per day for five days resulted
in a level of uterine adenocarcinoma at 18 months of age similar to that induced by
diethylstilbestrol (144). However, infant soy milk formulas do not appear to have
estrogenic effects in the reproductive tracts of mature mice (152), and a study of
four soy infant formulas revealed levels of genistein and daidzein resulting in a
daily dose rate of total isoflavones of approximately 3 mg/kg body weight (84),
much lower than that inducing cancer in neonatal mice.
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TABLE 1 Levels of isoflavones and lignans in various food sources. Values (in nanomoles per
gram dry weight) are taken from Reference 129 and were determined by isotope dilution gas
chromatography mass spectrometry with selected ion monitoring
Plant species (common name) Genistein Daidzein
Secoisolariciresinol Matairesinol
Soybean
993–3115 413–2205 <1–8
<1
Kidney bean
<1–19
<1–2
2–4
<1
American groundnut
4–30
<1
<1–2
<1
Chickpea
3–8
<1–8
<1
0
Pea
<1
<1
<1
<1
Lentil
<1
<1
<1
<1
Kudzu root
467
7283
<1
<1
Flaxseed
0
0
10,247
30
Sesame seed
<1
6
2
17
Sunflower seed
<1
<1
17
0
Peanut
2
1
8
<1
Wheat bran
<1
<1
3
0
Barley (whole grain)
<1
<1
2
Rye bran
0
0
4
5
0
Strawberry
0
0
33
<1
Cranberry
0
0
29
0
Blueberry
0
0
23
0
Raspberry
0
0
4
0
Red cabbage
<1
<1
4
<1
Broccoli
<1
<1
11
<1
Garlic
0
0
11
<1
Zucchini
0
0
23
<1
Carrot
0
0
10
<1
Beetroot
0
0
3
<1
Black tea
Trace
Trace
73
12
Green tea
Trace
Trace
75
5
Dietary supplements containing isoflavone phytoestrogens are now widely
available from health food stores and through the Internet. They are taken for various reasons, ranging from a belief in their anticancer potential, ability to alleviate a
range of postmenopausal problems (“natural” hormone replacement therapy), and
even breast enhancement. Major active compounds in these formulations are the
soy phytoestrogens, the isoflavonoid puerarin from Kudzu vine (Pueraria lobata),
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and formononetin from clovers and fenugreek (18). Several plant species that
are used as herbal medicines also contain isoflavonoids with estrogenic activity.
Clover has already been mentioned in this respect. Another example is the root of
licorice (Glychirriza echinata), which contains an estrogenic isoflavan (glabridin)
and isoflavene (glabrene) (176).
Black cohosh (Actaea racemosa syn. Cimicifuga racemosa) is a North American perennial plant that is being developed as a natural alternative to hormone
replacement therapy for treating menopausal symptoms. The active principle in
black cohosh was thought to be formononetin, the compound responsible for the estrogenic effects of clover. However, this was discounted in recent studies that failed
to detect this compound in a number of populations of the plant from throughout
the United States, and in several commercial preparations (94).
The levels of phytoestrogens in soybean seed can vary significantly depending on cultivar and environmental factors (54). More than 75% of the soybeans
currently grown in the United States are transgenic for resistance to the herbicide
glyphosate (RoundupTM). Glyphosphate inhibits biosynthesis of shikimate, a precursor of the amino acid phenylalanine from which isoflavones are derived. It was
recently shown that application of RoundupTM at various doses in the field does
not appear to affect phytoestrogen levels in soybean (51).
Biosynthesis
Genistein is biosynthetically the simplest isoflavonoid. It is a central intermediate in the formation of more complex isoflavonoids with roles in establishing
or inhibiting interactions between plants and microbes (48). Isoflavonoids are
formed by a branch of the flavonoid biosynthetic pathway, and originate from a
central flavanone intermediate [naringenin (40 ,5,7-trihydroxyflavanone) in genistein biosynthesis, liquiritigenin (40 ,7-dihydroxyflavanone) in daidzein biosynthesis] that is ubiquitously present in plants. For entry into the isoflavonoid pathway,
the flavanone first undergoes abstraction of a hydrogen radical at C-3 followed by
B-ring migration from C-2 to C-3 and subsequent hydroxylation of the resulting
C-2 radical. This reaction requires reduced nicotinamide adennine dinucleotide
phosphate (NADPH) and molecular oxygen, and is catalyzed by a microsomal cytochrome P450 enzyme [2-hydroxyisoflavanone synthase (2-HIS), loosely termed
isoflavone synthase (IFS)] (Figure 2B). The IFS reaction is stereoselective, and
(2R)-flavanones are not substrates. The resulting 2-hydroxyisoflavanone is unstable and readily undergoes dehydration to yield genistein or daidzein at acidic
pH. It has been suggested that a specific dehydratase enzyme catalyzes the 2hydroxyisoflavanone to isoflavone conversion in planta, and such an enzyme has
been purified from Pueraria lobata, although it has not been characterized at the
molecular level and is not necessary for production of isoflavones in transgenic
plants expressing IFS alone (92, 119).
Because of the lability and low abundance of IFS, the enzyme eluded molecular
characterization for many years. However, cDNAs encoding IFS have now been
cloned from soybean, licorice, and other species (8, 92, 170), largely aided by data
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mining of expressed sequence tag (EST) libraries. The enzyme converts liquiritigenin or naringenin to the corresponding 2-hydroxyisoflavanone in the presence
of NADPH. It will be interesting to analyze the reported nonlegume sources of
isoflavones listed in Table 1 for the presence of orthologs of legume IFS. If verified, this would significantly expand the number of known species in which IFS
enzymes have evolved.
Daidzein differs from genistein by lack of a hydroxyl group at the 5-position.
This hydroxyl group arises naturally from the head-to-tail condensations of
malonyl CoA residues during the formation of naringenin chalcone by chalcone
synthase. Loss of the 5-hydroxyl occurs during formation of the polyketide intermediate that cyclizes to give the chalcone, and is catalyzed by a specific NADPHdependent reductase incorrectly named “chalcone reductase” (Figure 2B). Chalcone reductase is not present in all plants. The presence of both daidzein and
genistein in soybean seeds and some clovers suggests that a significant proportion
of the polyketide intermediate can escape the action of chalcone reductase. In contrast, legumes such as alfalfa produce predominantly 5-deoxy-isoflavonoids such
as formononetin.
Formononetin from alfalfa and clovers, and biochanin A from chickpea, are
methylated on the 40 -position of the B-ring. The biosynthetic origin of this 40 methyl group has been a topic of controversy for many years, since it was observed that the only O-methyltransferase activity that could be detected in cellfree extracts from plant cultures producing formononetin catalyzed methylation
at the A-ring 7- rather than the B-ring 40 -position of daidzein or genistein (53,
71) (Figure 2B). Radiolabeling studies in alfalfa cell suspension cultures provided
convincing evidence to indicate that daidzein is not an intermediate in the biosynthesis of formononetin (120). This observation was made many years earlier in
studies on the biosynthesis of phytoalexins in elicitor-treated alfalfa seedlings
(45), and it was then proposed that O-methylation was an integral component
of the aryl migration reaction catalyzed by IFS (45). This idea lost support with
the later demonstration that the IFS reaction occurs with no requirement for Bring methylation (100). Two different explanations for the origin of the B-ring
methoxyl group have now been proposed. In both, the substrate for O-methylation
is the 2-hydroxyisoflavanone product of the IFS reaction formed prior to dehydration to yield the isoflavone (Figure 2B). In one model, based on studies of
the subcellular localization of the previously characterized alfalfa isoflavone 7-Omethyltransferase (IOMT) (120) and its reaction mechanism as determined from
its 3-dimensional crystal structure (205), the IOMT, when physically associated in
a complex with IFS, catalyzes 40 -O-methylation of one specific steroisomer of the
2-hydroxyisoflavanone intermediate (205). This model is supported by the observation that IOMT, which is an operationally soluble enzyme in healthy, unchallenged
alfalfa cells, relocates to the endoplasmic reticulum following treatments that induce the membrane-anchored IFS (120). In a second model, based on studies with
elicitor-treated licorice cell cultures, a separate and distinct enzyme from IOMT,
with no daidzein 7-O-methyltransferase activity, catalyzes the 40 -O-methylation
of 2-hydroxyisoflavanone (9). This latter O-methyltransferase has strong sequence
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homology to a pterocarpan A-ring O-methyltransferase implicated in the biosynthesis of the isoflavonoid phytoalexin pisatin in pea (153).
Conversion of isoflavones to their glucose and glucose-malonate conjugates
is catalyzed by glycosyl transferase and malonyl transferase enzymes that have
been characterized enzymatically, particularly from chickpea (101, 102), but not
yet at the molecular level. It is likely that some plant glycosyltransferases involved in conjugating compounds with phenolic hydroxyl groups may have relatively broad substrate specificity (186). For example, a glycosyltransferase from
Eucalyptus perriniana, a species that has not, to the author’s knowledge, been reported to produce isoflavonoids, efficiently converts daidzein to daidzin (daidzein
7-O-glucoside) (142). Likewise, enzymes in Arabidopsis thaliana, presumably involved in glycosylation of endogenous flavonols, can glycosylate genistein when
this “foreign” compound is produced transgenically (119).
Coumestrol, a member of the coumestan class of isoflavonoids (Figure 2A), is
derived from daidzein by a series of reactions that include hydroxylation/oxidation
at C2 and ring closure with loss of water between the hydroxyl on C4 of the
heterocycle (isoflavone numbering) and a hydroxyl on C20 of the B-ring (48).
Although this pathway was proven by radiolabelled precursor feeding studies, the
enzymes that catalyze the late stages of coumestan biosynthesis have yet to be
characterized at the molecular level.
Bioavailability and Metabolism
As indicated above, most flavonoids and isoflavonoids exist in the plant as glycosidic conjugates, generally located in the central vacuoles of the cells. Bioavailability of these true dietary components depends on relative uptake rates of conjugated
and free forms; hydrolysis of glycosides by gut bacteria or gut wall enzymes; further
metabolism, for example to glucuronides within the liver followed by enterohepatic
circulation; and excretion rate into urine and bile.
The malonyl glucosides of daidzein and genistein found in soybean are labile
and readily degraded to the nonacylated glucosides following cooking. The free
aglycones, but not the glycosides, are absorbed from rat stomach. However, once
in the small intestine, brush border lactase phlorizin hydrolase can effectively hydrolyze isoflavone glucosides (38). In humans, isoflavones appear in blood plasma
at a more rapid rate and at higher levels following oral administration of the aglycones compared with the glycosides, and genistein and daidzein, but not their glycosides, are readily transported across human intestinal epithelial cell monolayers
(171). Genistein attains higher plasma concentrations than daidzein when administered at the same level (164). This is because daidzein becomes more widely
distributed within the body. However, genistein has greater bioavailability than
daidzein, and overall bioavailability may increase if the compounds are ingested
as their glycosides (164). This is important because many chemical intervention
trials and in vitro studies in animal systems have utilized the free aglycones rather
than the naturally occurring glycosides.
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Although isoflavone ingestion results in increased levels of the parent isoflavone
in plasma and urine, the compounds are also further metabolized. Although soy
isoflavones exhibit estrogenic activity, their metabolite equol (Figure 2A), formed
from daidzein by intestinal bacterial activity, is significantly more estrogenic and
may be largely responsible for the physiological effects of isoflavone intake (165).
Equol and other isoflavone metabolites can also be delivered into the human diet
from cow’s milk (129). The levels of urinary equol in humans eating a soy-rich diet
can be approximately 100-fold higher than those observed in adults who consume
little soy products in their diet. Equol binds to both human ER forms (ERα and
ERβ), has high antioxidant activity, and is relatively stable. Conversion of soy
isoflavones to equol can be influenced by factors in the diet affecting microbial
populations, such as carbohydrate and dietary fiber levels (108, 168), a fact that
can complicate epidemiological and dietary intervention studies. There is now
evidence to suggest that the ability to produce equol is not the same in all humans,
and that some individuals do not have this ability, presumably because they are
host to a different population of intestinal microorganisms (165). In several studies,
excretion of equol only occurred in approximately 35% of cases, whether male or
female (168).
Biochanin A and formononetin are rapidly demethylated following ingestion,
giving rise to genistein and daidzein, respectively. In addition to metabolism by gut
bacteria, the demethylation reactions can also be catalyzed by a range of different
cytochrome P450 enzymes in the liver, with some catabolizing biochanin A but not
formononetin, and vice versa (181). The demethylated compounds can be further
hydroxylated by hepatic enzymes (181).
Soy isoflavones are also converted to a range of conjugates by mammalian
metabolism. Following chronic dosing of daidzein, its 7-glucuronide (54%), 40 glucuronide (25%), monosulfate (13%), free aglycone (7%), sulfoglucuronide
(0.9%), diglucuronide (0.4%), and disulfate (<0.1%) could be detected in human
urine using liquid chromatography electrospray ionization tandem mass spectrometry (29). O-desmethlyangolensin (Figure 2A) is also a metabolite of daidzein in humans, and a fluoroimmunoassay was recently developed for this compound (117).
The glucuronides retain weak estrogenicity and may be able to activate natural
killer cells that help combat cancer at physiological concentrations (203). Similar
metabolites of genistein to those listed above are found in rat and man. These
include genistein glucuronide, dihydrogenistein glucuronide, genistein sulphate,
dihydrogenistein, and 60 -hydroxy-O-desmethylangolensin (31). However, rat liver
slices or isolated hepatocytes only catalyzed the glucuronidation of genistein
(31).
Potential Health Beneficial Effects of
Isoflavonoid Phytoestrogens
HORMONE-DEPENDENT CANCERS The structural features of genistein and daidzein
confer ability to bind ER and sex hormone binding proteins, and isoflavone
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phytoestrogens can exert both estrogenic and antiestrogenic activity, the latter
by competing with estradiol for ER binding. Genistein and equol displace bound
estrogen and testosterone from human sex hormone binding globulin (SHBG), and
can affect the cellular levels of SHBG (151). Thus, genistein and other phytoestrogens could potentially affect clearance rates of androgens and estrogens and
therefore the availability of the hormones to target cells. Genistein and coumestrol exhibit significantly higher binding activities for ERβ than for ERα (129);
ERβ appears to be the prominent ER form in prostate secretory epithelium, brain,
urinary tract, and possibly also breast cells. This may be an important factor for
the apparent links between isoflavonoid phytoestrogen intake and reduced risk of
hormone-dependent cancers (129).
Significant correlations exist between an isoflavone-rich soy-based diet, urinary
isoflavone levels, and reduced incidence of breast cancer or mortality from prostate
cancer in humans (35, 64, 159). An early epidemiological study of Singapore
Chinese women that included 420 healthy controls and 200 with histologically
confirmed breast cancer indicated that soy consumption was directly correlated
with reduced risk of cancer (109). The positive effects of a soy diet appeared
to be dietary rather than genetic because Asians who immigrate to the United
States and adopt a Western diet are at higher risk of breast and prostate cancers.
Similar observations have been reproduced in many, but not all, subsequent studies
undertaken up to the present day, and the effects may also be in part attributed to
high dietary lignan levels (35). Based on knowledge of diet and urinary excretion
levels of daidzein, genistein, and equol in Japanese compared with American or
European subjects, the isoflavonoids found in soy products were proposed to be
the agents responsible for reduced cancer risk. However, cancer risks are also low
in some Asian populations that do not have a high soy diet (64), and a study with
postmenopausal Dutch women failed to reveal a link between dietary isoflavone
levels and breast cancer risk (42).
Neonatal administration of genistein effectively protects against chemically
induced mammary tumors in rats (59). The protective effects include increased
latency, reduced incidence and multiplicity of tumors, and more rapid maturation
of undifferentiated end buds to differentiated lobules (33). These effects, which
appear to be ER mediated, are associated with increased epidermal growth factor
receptor (EGFR) and progesterone receptor (PR) expression in the prepubertal rat
mammary gland (33). Thus, genistein may induce early mammary gland differentiation resulting in a less active EGF signaling pathway in adulthood that, in
turn, suppresses development of mammary cancer (106). Although the author is
aware of no reported clinical trials documenting effects of controlled dietary supplementation with genistein on breast cancer incidence in humans, a high soy diet
containing up to 45 mg of isoflavones per day can cause changes in the menstrual
cycle that may help reduce cancer risk. In contrast to the results of these studies,
dietary feeding of supraphysiological concentrations of daidzein to female rats
neither caused significant toxicity to the reproductive tract nor provided protection against chemically induced mammary cancer (107). In studies in which rats
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were fed a standardized soy extract instead of pure isoflavone, chemically induced
mammary adenocarcinomas took longer to develop than in control animals, but at
the end of the study no difference in tumor multiplicity or incidence was observed
between treatment and controls (62).
Isoflavonoid phytoestrogens show complex effects on the growth of breast cancer cells grown in vitro. Biochanin A has a multiphasic activity on human mammary
carcinoma cells, being stimulatory to growth at very low concentrations (less than
10 µg/ml), cytostatic at intermediate concentrations (40 µg/ml), and cytotoxic
at higher concentrations (81). These effects appear to be ER dependent, and ER
mRNA levels paralleled the growth rates of the cells at the different biochanin
A concentrations (80). In a separate study, differential effects of genistein and
coumestrol were observed in relation to ER and PR levels in the same mammary
carcinoma cell line, with coumestrol behaving as an ER agonist and genistein
showing features of a selective ER modulator (46). This latter activity might involve isoflavone-mediated selective recruitment of coregulatory proteins to ERβ
to trigger transcriptional pathways (13).
Biphasic growth stimulation (measured as DNA synthesis) followed by inhibition of human mammary carcinoma cells was also observed as a function of
increasing concentrations of a number of other (iso)flavonoid compounds, including coumestrol, genistein, apigenin, luteolin, kaempferol, and the lignan enterolactone (see below) (188). This may be a concern, particularly for women who
already have initiated breast cancer, in view of low but significant phytoestrogen
concentrations in some diets (188). One report suggests that low concentrations
of genistein may antagonize the effects of the structurally related compound tamoxifen (Figure 2A) (40), a drug used as a chemopreventive for women at high
risk for breast cancer. However, at high concentrations, such as those reached with
a soy-rich diet, genistein is a strong cytotoxic agent against breast cancer cells
with a mechanism independent of ER (123). Dietary intake levels might be the key
to isoflavone phytoestrogens’ mode of action and the balance between risk of, or
chemoprevention from, breast cancer (123).
Genistein is unique among a number of flavonoid and isoflavonoid compounds
tested in having both strong estrogen agonist activity and strong growth inhibitory
activity against breast cancer cells (202). In contrast, equol has strong estrogen
agonist activity but little growth inhibitory activity. Genistein is more effective
in inhibiting growth of non-neoplastic human mammary cell lines than it is in
inhibiting growth of mammary cancer cells (167). This supports the notion that
early exposure to genistein may be important for breast cancer chemoprevention.
Prostate cancer is the second most frequent cause of cancer-related deaths in
men in the United States, and there is no effective therapy for the disease once
it becomes metastatic. Inverse relationships have been observed between high
phytoestrogen intake and incidence of and mortality from prostate cancer (7), and
prostate cancer cell growth in vitro and in vivo, and levels of androgen-related
prostate-specific antigen, are all decreased by administration of genistein (37,
132). High concentrations of genistein do not appear to be toxic to the rat prostate
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(60), although they may induce inflammation (prostatitis) (105). In a case control
study of 83 prostate cancer cases and 107 controls carried out in the United States
between 1996 and 1998, dietary intake of coumestrol and daidzein appeared to be
more significantly related to reduced prostate cancer risk than did dietary intake
of genistein (173). In the same study, there appeared to be a positive relationship
between intake of the phytosterols campestrol and stigmasterol and prostate cancer
(173). Daidzein only exhibited weak inhibitory effects on growth of benign and
malignant human prostate epithelial cells, but its metabolite equol had potent
inhibitory effects at micromolar concentrations (72). Thus, conversion of daidzein
to equol may be an important factor in dietary prevention of prostate cancer.
In addition to effects on breast and prostate cancers, genistein
and related isoflavones also inhibit cell growth and/or development of chemically
induced cancers in stomach, bladder, lung, and blood. Inhibition of the growth
of human stomach cancer cell lines in vitro by genistein and biochanin A involves stimulation of a signal transduction pathway leading to apoptosis (198).
When these cancer cells were transplanted into mice, biochanin A, but not genistein, significantly inhibited tumor growth. Genistein strongly inhibits growth of
leukemia cells when targeted to them by linkage to a monoclonal antibody (183),
and a prenyl isoflavone derivative (ipriflavone) was developed as an oral treatment
for acute leukemias (147). The prenyl group might help target the isoflavone to
hydrophobic sites of action.
A preliminary study, based on a multiethnic population-based case-control analysis of thyroid cancer conducted in the San Francisco Bay area, led to the conclusion that there may be a link between an isoflavone-rich diet and reduced risk of
thyroid cancer (79). The study involved over 800 white and Asian women, both
pre- and postmenopausal. However, the authors were cautious in interpreting the
data, and indicated that further trials would be necessary before firm conclusions
could be made.
There have been claims for a protective effect of isoflavones on colon cancer,
but these are somewhat conflicting (3). At low concentrations, genistein induces
the phase II detoxifying enzyme quinone reductase in colonic cells; biochanin
A and coumestrol also have this ability, albeit less effectively, but daidzein and
formononetin are inactive (192). Induction of a carcinogen detoxifying system
could provide a partial explanation for anticancer effects of phytoestrogens.
In spite of the large number of studies supporting cancer chemoprevention
by genistein, some studies have suggested a potential for opposite effects. These
include increased numbers of carcinogen-induced aberrant crypt foci in the colons
of rats fed genistein (66) and induced structural chromosome aberrations in human
peripheral lymphocytes (104).
OTHER CANCERS
POSTMENOPAUSAL AILMENTS Estrogen deficiency in postmenopausal women can
lead to overall bone loss resulting from increased bone resorption and decreased
bone formation. This condition, referred to as osteoporosis, is a major public health
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problem. There has therefore been considerable interest in the reports suggesting
that soy isoflavones can attenuate bone loss associated with estrogen deficiency.
Isoflavone phytoestrogens stimulate osteoblastic bone formation, inhibit osteoclastic bone resorption (52), and prevent overall bone loss in ovariectomized rats
or mice (55, 85). Suggested mechanisms for these effects include stimulation of
proliferation of osteoblast (bone forming) cells and protection of such cells from
oxidative damage, and increased apoptosis of osteoclast (bone destroying) progenitor cells (112, 154, 196).
The above effects in animal model systems were confirmed in several controlled
clinical trials with postmenopausal women, which indicated that an isoflavonerich soy diet may attenuate bone loss in the lumbar spine and hip (12, 131, 140),
and that this effect is due to isoflavones rather than to soy protein (12). However, such effects were not observed in premenopausal women (131). A recent
study reported that habitual tea drinking may also increase bone mineral density in the lumbar spine and hip (194). It is possible that strong flavonoid antioxidants such as epicatechin gallate found in tea are responsible for this effect. Isoflavones also possess antioxidant properties, and there may therefore
be more than one mechanism whereby plant natural products protect against
osteoporosis.
Phytoestrogens may be effective in reducing other symptoms of estrogen depletion in postmenopausal women, such as hot flushes, night sweats, and vaginal
dryness (11, 16). A recent study carried out in Spain on 190 postmenopausal women
reported statistically significant improvements in hot flashes, with no negative side
effects, in response to dietary supplementation with soy isoflavones administered
through commercially available capsules (10). Red clover extracts are popular dietary supplements, with many perceived beneficial effects including reduction of
postmenopausal symptoms (143). However, some studies with clover and soy have
failed to confirm efficacy (16, 61). In one study, the group of women taking the
dietary soy supplement reported increased incidence of insomnia (16). A recent
review of 74 studies addressing effects of phytoestrogens on postmenopausal ailments concluded that, although these compounds may indeed have some degree of
efficacy, this falls well short of that obtained from traditional hormone replacement
therapy (68).
The potential beneficial effects of soy isoflavones on breast cancer risk have
also been questioned in studies that specifically target postmenopausal women. For
example, a study that included 88 breast cancer cases and 268 controls, selected
from a large group of postmenopausal women participating in a breast cancer
screening program in the Netherlands, failed to demonstrate a significant correlation between phytoestrogen intake (assessed as urinary phytoestrogen level) and
cancer risk (182).
CARDIOVASCULAR DISEASE Results of epidemiological studies suggest that high
dietary intake of isoflavones and/or flavonols may contribute to a low incidence of
heart disease in Japanese women. Effects of isoflavones on cardiovascular health
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may result from inhibition of low-density lipoprotein (LDL) oxidation (178, 179),
an effect that may be enhanced by food sources rich in vitamim C (82); inhibition of
proliferation of aortic smooth muscle cells, as concluded from a study with strokeprone spontaneously hypertensive rats (148); and maintenance of the physical
properties of arterial walls (185).
Postmenopausal women have increased risk of cardiovascular disease due in
part to elevated cholesterol levels accompanying the loss of endogenous estrogen
secretion. In premenopausal women, dietary genistein appears to improve plasma
lipids (resulting in lowered LDL cholesterol), the ratio of total cholesterol to highdensity lipoprotein (HDL) cholesterol, and the ratio of LDL to HDL cholesterol
(133). Positive effects on cardiovascular risk profile were also concluded from a
study of more than 900 postmenopausal women (99), although such effects were
not observed in a separate study with a smaller sample size (43). The protective
effects of soy phytoestrogens appear most significant in individuals with initially
elevated cholesterol levels; phytoestrogen intake may have less or no effect on
individuals with normal cholesterol levels (76, 178). On the basis of such findings,
the U.S. Food and Drug Administration has approved use of food health claims
for intakes of 25 g isoflavone-rich soy protein per day for the reduction of blood
cholesterol levels (88). This would equate to approximately 6 mg of genistein
per day. In a study on potential toxicology of phytoestrogens, the no-observedadverse-effect level for genistein was estimated to be 120 mg/kg per day in rats
(146). In rats, the hypocholesterolemic effect of a soy diet may involve interactions
between the isoflavones and soy protein (149), whereas in cholesterol fed rabbits,
attenuation of atherosclerosis by isoflavones does not require the presence of soy
protein.
A comparison of the progression of atherosclerosis in ovariectomized cynomolgus monkeys treated with equine estrogen or soy isoflavones indicated that the plant
products, although providing protection, were less effective than mammalian estrogen (30). Finally, soy intake may have beneficial effects with respect to obesity
(with links to cardiovascular disease) and diabetes, although it is not clear whether
this is due to isoflavones or other components (20).
COGNITIVE FUNCTION Depletion of estrogen at menopause may be associated
with increased risk of neurodegenerative diseases, and estrogen replacement therapy improves episodic and semantic memory in postmenopausal women. Remarkably, in one study a high soy diet improved memory within weeks in both young
male and female human volunteers (57). A series of studies on brain structure,
learning, memory, and anxiety in rats led to the conclusion that high consumption
of dietary phytoestrogens over a relatively short time period can significantly alter
the volume of sexually dimorphic brain regions, increase anxiety, and improve
learning and visual-spatial memory in females but not in males (115). Generally,
males outperform females in a maze test, but this sexual dimorphism was reversed
when the animals were placed on a phytoestrogen-rich diet (122). When the male
rats were “feminized” by treatment with the androgen receptor blocker flutamide,
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they then responded to phytoestrogen in a similar manner to females (121). The
effects of phytoestrogen on sexually dimorphic memory performance were not associated with changes in brain aromatase levels (115). These studies are exciting
in view of the potential importance of estrogens in brain and neural disorders such
as Alzheimer’s disease, especially in women.
A recent study indicated that genistein has comparable ERβ-mediated antiapoptotic properties to 17β-estradiol in primary cortical neurons (118). Understanding
more about how isoflavone phytoestrogens may affect cognitive function will be
an exciting goal for the future.
REPRODUCTION AND FERTILITY The phytoestrogens were first identified by their
effects on fertility in sheep grazing high-isoflavone containing clovers. Subsequently, it was suggested that decreasing fertility rates in some human populations
may be a result of exposure to environmental estrogens. However, little experimental evidence supports this concept. Genistein and the flavanone phytoestrogen
8-prenylnaringenin (see below) actually increased the fertilizing ability of mouse
sperm in vitro at submicromolar concentrations, apparently by an ER-independent
mechanism (1). In male rats, long-term exposure to genistein in utero, during
lactation, and for up 130 days post gestation had no apparent adverse effects on
gametogenic function and sperm count (158). In human males, a two-month exposure to high dietary isoflavone levels likewise had no negative impact on semen
function (137).
Elevation of plasma isoflavone levels resulting from inclusion of soy milk in
the diet correlates with elevated SHBG levels in postmenopausal women (151).
However, a randomized double-blind trial with 34 premenopausal women fed
either 100 mg of isoflavones per day or placebo over a one year period indicated
no alterations in menstrual behavior (127). A six-month intake of phytoestrogens
had no effect on endometrial histology in a group of 19 postmenopausal women
(16). However, soy isoflavones have been reported to reduce the incidence of
menstrual-linked migraines that have been associated with fluctuations in estrogen
and progestin levels (25).
On balance, the evidence suggests that isoflavone phytoestrogens may represent
a safe and natural dietary supplement for alleviating many ailments that affect older
women. However, significant physiological effects may result from exposure to
isoflavone phytoestrogens during development of the female reproductive system,
at least as demonstrated in animal models. Thus, neonatal exposure to genistein
induces ERα expression and multioocyte follicles in the maturing mouse ovary
(89), and use of DNA array analysis indicated that genistein causes similar changes
in a specific set of gene transcripts to those caused by an estradiol derivative in
developing rat uterus (141). Japanese infants at birth can exhibit high levels of
isoflavone phytoestrogens passed through the placenta from the mother (6), and,
although it has been suggested that this could be an important factor for reduced
breast cancer risk later in life, there are also concerns about potential deleterious
effects.
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INFLAMMATION AND IMMUNITY Licorice root is commonly used in Chinese traditional medicine for treating inflammation, allergies, and asthma. Although
licorice contains bioactive triterpenes and chalcones, the isoflavonoid licoricidin
was recently shown to inhibit lyso-platelet activating factor (PAF) acetyltransferase, making it a strong candidate for the active antiallergenic principle (21).
An isoflavone phytoestrogen–rich diet has also been shown to reduce inflammatory markers in a guinea pig model of asthma, although this was accompanied by
potentially detrimental leakage of protein into the airspace of the lungs following
challenge with an aerosol containing ovalbumin, to which the guinea pigs had been
previously sensitized (155).
Additional concerns about the safety of infant exposure to soy-based formulas
have been raised based on the results of experiments addressing effects on the
thymus. Injection of genistein into ovariectomized adult mice at a concentration
that produced similar serum genistein levels to those reported in soy-fed human
infants resulted in significant loss of weight of the thymus gland, apoptosis of thymocytes leading to drastically reduced thymocyte numbers, changes in spleen cell
numbers, and reduced humoral immunity (199). In a subsequent study, the same
treatment reduced cellular immunity (200). In both studies, the effects of genistein
were concluded to occur through ER-dependent and independent pathways. The
implications of these studies for the development of the human immune system are
not yet understood. Essentially opposite effects of genistein on spleen and thymus
have been reported in studies with rats (70), and studies with adult humans have
suggested that exposure to genistein might improve immune surveillance associated with increased levels of interleukin-6, at least in females (90). No significant
effects were seen in these studies with males, and other inflammatory markers and
cytokines were unaffected by genistein intake (90). Finally, genistein has been
reported to increase host immunity in a mouse tumor model system, and this was
proposed to involve increases in the activities of cytotoxic T cells and natural killer
cells (69).
Metabolic Engineering of Isoflavone Phytoestrogens
As the entry point enzyme into isoflavonoid biosynthesis, IFS is the key step for
engineering isoflavone production into nonleguminous plants that lack the pathway. Such a metabolic engineering strategy could serve two functions: to validate
or invalidate the concept of health promotion by isoflavones by providing near isogenic material containing different concentrations of isoflavones in their naturally
bioactive forms for animal dietary intervention trials, and if efficacy is validated, to
provide new, value-added food crops for human health enhancement. To establish
proof of principle for metabolic engineering of isoflavones, soybean IFS was introduced into Arabidopsis thaliana, corn, and tobacco (92, 119, 201). Free genistein
does not accumulate but is in Arabidopsis converted to a series of glucose and rhamnose conjugates, including the rhamnoglucoside (119), reflecting the glycosylation
pattern of the endogenous Arabidopsis leaf flavonols, kaempferol and quercetin.
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The above transgenic lines only accumulate genistein conjugates to very low
levels. This could be a result of the IFS activity level, substrate availability, substrate
channeling, or product turnover. Various strategies have been taken to address the
problem of substrate availability for engineered IFS. Upregulation of flavonoid
synthesis in maize Black Mexican Sweet cell cultures expressing soybean IFS
was achieved by expression of a chimeric transcription factor (CRC) containing
the maize C1 and R transcription factor coding regions; this led to low levels
of genistein production, from undetectable levels in the absence of CRC expression (201). Overexpression of chalcone isomerase in Arabidopsis expressing IFS
led to a threefold increase in flavonol levels, but this was not accompanied by
a corresponding increase in genistein conjugates. Likewise, genistein production
in transgenic Arabidopsis was not increased in the pap1-D genetic background
in which anthocyanin production is strongly upregulated (119). Thus, metabolic
channeling through the endogenous pathways of flavonoid biosynthesis in Arabidopsis results in limitations to flux through the introduced IFS. This is explained
by competition between flavanone 3-hydroxylase (F3H, the entry point into the
flavonol pathway) and IFS for their common substrate naringenin. This idea is supported by the high-level production of genistein conjugates following expression
of soybean IFS in the tt3/tt6 mutant of Arabidopsis, which lacks F3H (119).
Although Medicago species such as alfalfa possess IFSs capable of producing genistein, this compound does not naturally accumulate in alfalfa. Rather, the
5-deoxy class of isoflavonoids is produced, due to the coexpression of IFS and
chalcone reductase. These compounds, primarily formononetin and medicarpin
conjugates, are produced constitutively in roots but not in leaves, but the corresponding aglycones accumulate in many tissues following microbial challenge.
In contrast to the situation in Arabidopsis, ectopic expression of IFS under the
CaMV 35S promoter leads to high levels of genistein accumulation in alfalfa leaves
(B. Deavours & R.A. Dixon, unpublished results). This might result from the fact
that the constitutively formed flavonoids in alfalfa are primarily flavones rather
than flavonols (83), such that competition between IFS and F3H is not an issue.
Metabolic engineering studies for human health enhancement are in their infancy, and their relevance for the marketplace will be dictated by the emerging
evidence for or against potential advantages and, of course, safety. In soybean,
which is already marketed as a health-friendly food in view of its natural phytoestrogen content, genetic modification has been targeted toward obtaining more
reproducible levels of daidzein and genistein, which vary depending on environmental conditions (77).
Phytoestrogens and Sex Determination in Plants
Information from plant EST and genomic sequencing projects to date would tend
to suggest that plants do not contain functional orthologs of mammalian ER. Two
low-abundance EST sequences from the model legume Medicago trunctula have
homology to ERα and ERβ respectively, but similar sequences were not found
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in other plant EST databases. The possibility that phytoestrogens might possess
“estrogenic” functions within the plant is, therefore, perhaps fanciful. Nevertheless,
preliminary data from the “osage orange” (Maclura pomifera, Moraceae) indicate
that male and female branches of this dioecious species contain very different
levels of a compound(s) able to activate human ER in yeast (124). M. pomifera fruit
contain up to 6% dry mass of the di-prenylated isoflavone derivatives pomiferin and
osajin, with smaller amounts of prenylated isoflavones and flavones with known
in vitro ER activation activity in other parts of the plant (111). Whether the ERactivating compound(s) of M. pomifera truly play a role in sex determination in
this species has not been determined.
FLAVONOID PHYTOESTROGENS
Occurrence
Although flavonoids occur ubiquitously in higher plants, their potential as phytoestrogens has received less attention than that of the isoflavonoids. Flavonoids typically either exhibit negligible or low estrogenic activity, particularly those found
in plants that are a significant component of the human diet. Although weakly estrogenic flavonoids do occur in edible plants that are also used medicinally, such as
mugwort (Artemisia vulgaris, used for inducing regular menstruation) (110), most
interest in the flavonoids is associated with their antioxidant activity and potential
roles in reducing heart disease (74, 157). Nevertheless, some flavonoids, particularly flavones, are better inhibitors of the aromatase cytochrome P450 that converts
androgens to estrogens than are the isoflavones (93), and could therefore potentially
affect estrogen levels in women. Limited reports suggest that commonly occurring flavonoids, such as the anthocyanidin pigments found in flowers and leaves
(e.g., red cabbage) can exhibit ER-dependent responses in model systems (161),
though they have very weak ER-binding activity. Because these compounds are
often ingested in large quantities, their effects may warrant further investigation.
8-Prenylnaringenin (Figure 2B), a compound produced by hops (Humulus lupulus) and found in relatively small quantities [less than 20 µg/L, (177)] in some
beers, is a much more potent flavonoid phytoestrogen. This molecule exhibits
ER-mediated activity in mammalian cells at a concentration of approximately
1.0 µM (204), at least an order of magnitude lower than that of the parent flavonoid
naringenin. Other prenyl-substituted flavonoids such as 6-prenylnaringenin, 6,8diprenylnaringenin, and 8-geranylnaringenin also exhibit estrogenicity. The
lipophilic substitution may help target 8-prenylnaringenin to ER or estrogenmetabolizing enzymes in vivo.
Along with a range of related prenylated chalcones and bitter acids, 8-Prenylnaringenin accumulates in the lupulin glands (peltate trichomes) found on the
underside of the bracts of the female hop flowers used for flavoring beer. The
compound when extracted from hops is a racemic mixture of both (+)- and (−)enantiomers, both isomers showing similar binding characteristics to ER (22).
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8-Prenylnaringenin is therefore most likely formed by nonenzymatic cyclization
of prenylated naringenin chalcone, as the enzyme chalcone isomerase produces
only the (−)-flavanone. Indeed, the prenylchalcone xanthohumol (a derivative of
the prenylchalcone shown in Figure 2B, but with a methoxy group at position 60 of
the A-ring), which has little estrogenic activity (135), is present in the lupulin glands
at much higher concentrations (two orders of magnitude) than the prenyl naringenins (22). The prenylation reaction uses dimethylallylpyrophosphate (DMAPP)
as prenyl group donor. Direct enzymatic prenylation of naringenin can occur in
plants, for example during biosynthesis of sophoroflavanone G (8-lavandulyl-20 hydroxynaringenin) in Sophora flavescens (197). It is also possible that prenylation of naringenin, or conversion of prenylated chalcone to prenylnaringenin, can
occur in beer during storage (75). Because of the increased bioactivity of prenylated flavonoids compared with the nonsubstituted compounds, flavonoid/chalcone
prenyltransferases represent an important class of genes for metabolic engineering
in plants.
Because of the differential solubility of prenyl naringenins and the hop bitter
acids, spent hops (the plant material after extraction of the acids) represent an
enriched source of phytoestrogens for herbal supplements or functional foods
(22).
Resveratrol (trans-3,5,40 -trihydroxystilbene) (Figure 2B), a compound found in
grapes (and therefore wine) and peanuts, is not strictly a flavonoid but is included
here because it is biosynthetically related to the true flavonoids. The enzyme
stilbene sythase (SS) is a polyketide synthase that is evolutionarily related to the
chalcone synthase (CHS) of flavonoid biosynthesis. The same initial polyketide
intermediate is formed by condensation of one molecule of 4-coumaroyl CoA and
three molecules of malonyl CoA, but in SS the final cyclization is accompanied
by a decarboxylation reaction. SS genes were cloned from a number of species,
and are often phylogenetically indistinguishable from CHSs based on amino acid
sequence alone (162). Structural features that distinguish CHSs from SSs have now
been determined based on knowledge of the three-dimensional crystal structure of
CHS (56) and rational site-directed mutagenesis experiments (91, 174).
Potential Health Beneficial Effects
8-Prenylnaringenin tests positive in a number of in vitro assays that assess potential
health beneficial effects (172). For example, it inhibits the growth of human breast
cancer cells in vitro, inhibits bone loss in ovariectomized mice (138), and inhibits
the CYP1A2 cytochrome P450 involved in procarcinogen activation (73). It binds
similarly to both ERα and ERβ, and its estrogenic activity in vitro is greater than
that of genistein, daidzein, or coumestrol. However, tests in mice have shown
that estrogenic effects of 8-prenylnaringenin supplied in drinking water require
concentrations approximately 500-fold higher than those found in beer (135).
Likewise, the levels of 8-prenylnaringenin present in products advertised for breast
enhancement are insufficient to exhibit estrogenic activity at the level of the uterus
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based on studies in mice (32). However, there has been some concern expressed
about unrestricted intake of hop-based products with high phytoestrogen activity
(135), and it has even been suggested that 8-prenylnaringenin might be responsible
for menstrual disturbances in female hop workers (136).
Much of the interest in resveratrol centers on its perceived effects on cardiovascular health, and it has been postulated that its presence in red wine may be one
factor in explaining the so-called “French paradox” (67), whereby a population
with relatively high dietary fat intake has a lower than average incidence of cardiovascular disease. Resveratrol has also been reported to exhibit anticancer activity
(87). It is a weak ER agonist/antagonist with equal binding activity for ERα and
ERβ (24), but the relative extent of its in vivo estrogencity is cell type–dependent
(67). Other activities of resveratrol consistent with phytoestrogenic properties and
positive effects on cardiovascular health include enhancement of expression and
activity of endothelial nitric oxide synthase (187), inhibition of platelet activation
(49), and suppression of oxidative DNA damage (139). However, a study concluded
that resveratrol is inactive in immature rat uterotrophic assays and questioned the
degree of ER binding by resveratrol and its significance for the compound’s cardiovascular activity (14).
LIGNAN PHYTOESTROGENS
Occurrence
Lignans are defined as dimeric phenylpropanoid (C6-C3) compounds, mostly linked
8-80 (Figure 3), although many other linkage types are now encompassed within
the term lignan. They are a common, structurally diverse class of plant natural
product, and are widely distributed throughout the plant kingdom, where they
function primarily in plant defense (116). Their major dietary sources are the
outer layers of cereals and grains, with rye and flaxseed among the most important
(Table 1). Analytical approaches for lignans include HPLC (103) and GC (129),
with typical values of around 370 mg secoisolariciresinol per 100 g dry weight for
flaxseed (2). Berries and garlic are also dietary sources of lignans (2) (Table 1).
Similar to isoflavones, the lignans exist in the plant as glycosides stored in the
vacuole, and are converted to active phytoestrogens by microflora in the proximal
colon. Thus, the glycosides of secoisolariciresinol and matairesinol are effectively
converted to the so-called “mammalian lignans” enterodiol and enterolactone, respectively (Figure 3), and enterodiol can also be converted to enterolactone. It
was recently shown that pinoresinol and lariciresinol (precursors of secioisolariciresinol), as well as syringaresinol, can also be metabolized to enterolactone (3).
Thus, many lignans commonly found in whole grain products can be converted to
phytoestrogens.
Serum enterolactone levels are elevated in men and women following provision
of a diet rich in whole grains compared with refined grain foods (86). Consistent
with the production of enterolactone by microbial metabolism of matairesinol
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and secoisolariciresinol glycosides, taking oral antibiotics can significantly reduce
levels of enterolactone (95).
Biosynthesis
The “monolignol” coniferyl alcohol, which is also a precursor of polymeric lignins,
is the main building block of many lignans. Studies on the mechanism whereby
two coniferyl alcohol molecules are linked to give the initial lignan moiety have
revealed a unique biochemical mechanism for engendering stereospecific free radical coupling. Many lignans exist in a particular plant, or organ of that plant, in a
single steroisomeric form. However, the dimerization reaction occurs through free
radical coupling, initiated by single electron oxidation catalyzed by an oxidase such
as a peroxidase or laccase (116). Such reactions, which also occur during lignin
biosynthesis, are not stereochemically specific. The breakthrough in understanding
lignan coupling came when Norman Lewis’s group demonstrated the presence of a
novel, noncatalytic protein present in the cell wall fraction of Forsythia intermedia
stems. This so-called dirigent protein (Figure 3) was capable of determining the
stereochemical course of the coupling of two molecules of coniferyl alcohol in the
presence of an oxidase (36). In the absence of the dirigent protein the resulting
lignan was a mixture of the two potential steroisomers, whereas the dirigent protein alone had no catalytic activity (36). Dirigent proteins are encoded by a large
gene family with differential expression patterns in western red cedar, suggesting
that there is considerable tissue and environmental specificity to the monolignol
coupling reaction(s) (98).
The postcoupling steps in lignan biosynthesis vary greatly depending on the particular compound under consideration. In the flaxseed lignans, the major dietary
sources of “mammalian lignans,” the product of stereoselective monolignol coupling is (−)-pinoresinol (Figure 3). This is then converted to secoisolariciresinol
via a two-step reduction. The enzyme involved, pinoresinol/lariciresinol reductase, is a member of a large gene family that includes isoflavone reductases from
legumes (63). Finally, secoisolariciresinol is converted to the corresponding diglucoside (116). The availability of the genes encoding the enzymes of monlignol
coupling and downstream reduction make genetic modification of lignan content
a possibility.
Potential Health Beneficial Effects
As with the isoflavone phytoestrogens, evidence for health-promoting effects of
lignans, particularly in the areas of hormone-dependent cancers and cardiovascular
disease, has come from both epidemiological and chemical intervention studies
(35, 79, 99), particularly in Scandinavia where the population consumes a diet rich
in whole grain bread and grain fiber (5). In a review published in 1998, Adlercreutz concluded that breast cancer can be associated with low lignan levels in the
United States, Finland, Sweden, and Australia (7). The epidemiological data on
the relation between lignan consumption and prostate and colon cancers, as well
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as coronary heart disease, were too limited to draw clear conclusions (7). In contrast, several studies on the effects of enterodiol and enterolactone on mammalian
cancer cell lines in vitro have supported potential roles in cancer chemoprevention
(191).
More recent studies have left a less clear picture of the link between lignan
consumption and breast and prostate cancers. For example, a study with postmenopausal Dutch women (88 breast cancer cases and 268 controls), in which
urinary enterolactone levels were determined over a nine-year period prior to developing breast cancer, failed to reveal a link between urinary mammalian lignan
levels and breast cancer risk (42). Similarly, a large-scale study determining serum
enterolactone levels in nearly 800 men who subsequently developed prostate cancer and over 2500 control men failed to support the hypothesis that high-circulating
enterolactone levels protect against prostate cancer (169).
Evidence exists in humans that associations with reproductive risk factors for
breast cancer differ according to cytochrome P450c17A (CYP17) genotype (130).
In a recent study in which women in the highest tertile of dietary lignan intake
had reduced breast cancer risk, the effect was more significant for premenopausal
women with at least one CYP17A2 allele (130).
Dietary supplementation with 10% flaxseed to nude mice with established human breast tumor xenografts reduced tumor growth and metastasis (34). This was
associated with decreased extracellular levels of vascular endothelial growth factor
(VEGF), an important factor in angiogenesis and therefore cancer spread.
In a study in which hypercholesterolemic postmenopausal women that were
put on a classical hormone replacement therapy were compared with those put on
a flaxseed-rich diet, only the group on hormone replacement therapy had significantly improved cholesterol profiles and favorably modified markers related to
cardiovascular health (113).
OTHER CLASSES OF PHYTOESTROGENS
Although not common dietary components, several other compounds with estrogenic activity are consumed by humans in herbal remedies. These include
isoflavonoids and chalcone (isoliquiritigenin) from licorice root (discussed above).
Rhubarb contains the phenylbutanone glucoside lindleyin, which binds to ERα
and may be responsible for the biological effects of rhubarb extracts (184). Deoxymiroestrol is a potent phytoestrogen from the “rejuvenating” folk medicine
Kwao Keur produced in Thailand from Pueraria mirifica (27). Ginseng contains
several bioactive triterpenoid compounds, among which the glycoside ginsenoside stimulates proliferation of human breast cancer cell lines in an ER-dependent
manner, and can activate ER element reporter gene constructs in transfected HeLa
cells (26). The corresponding aglycone exhibited no significant activity. Extracts
from Polygonum, Cassia, Aloe, and Rheum species enhance cell proliferation in
estrogen-sensitive human breast cancer cell lines (128), and this is due to the
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activity of anthraquinones. Emodin and 2,6-dihydroxyanthraquinone were among
the most potent, and also inhibited 17β-estradiol binding to human ERα and ERβ
(128).
Fresh corn products contain a mitogenic compound with estrogenic activity.
This was discovered when corn cob bedding was shown to disrupt estrus cycling
and sexual behavior in rats. The compound stimulates proliferation of ER-positive
and ER-negative breast cancer cells, but does not compete for binding to ER
(125). The activity copurifies with an isomeric mixture of linoleic acid derivatives
with a tetrahydrofuran ring and two hydroxyl groups (THF-diols) that include
9, (12)-oxy-10,13-dihydroxystearic acid, and 10, (13)-oxy-9,12-dihydroxystearic
acid, and these compounds disrupt estrus activity in rats at concentrations 200-fold
lower than classical phytoestrogens (126). According to one report, fresh corn cobs
and corn tortillas also contain these compounds, suggesting potential for human
exposure (125).
Safflower (Carthamus tinctorius L.) seeds have long been clinically used in
Korea to promote bone formation and prevent osteoporosis (97), and dietary supplementation with safflower seeds partially protected against ovariectomy-induced
bone loss in rats (97). Preliminary studies indicated that this activity was associated
with the polyphenolic fraction of the safflower seed, but the specific components
responsible were not identified.
REASSESSING THE HEALTH BENEFICIAL EFFECTS
OF PHYTOESTROGENS
Clearly, firm data unequivocally linking phytoestrogen intake to a reduced risk of
disease are yet to be obtained for many diseases. This contrasts with the growing
use of dietary supplements containing phytoestrogens. In two critical reviews (3,
4), Adlercreutz summarizes the status of the current body of work on phytoestrogens and human health. He concludes that there is a significant relationship
between a high-isoflavone diet and reduced risk of breast cancer, but that this only
holds if isoflavones are consumed throughout life, or at least before and during
adolescence. There is also an inverse relationship between low plasma enterolactone levels and breast cancer. An isoflavone-rich diet may protect against prostate
cancer, but more research is needed. There is no evidence to date indicating that
isoflavones protect against colon cancer, or that high and continuous exposure
to phytoestrogens may present an increased risk of breast cancer. Phytoestrogens
have great potential for cardiovascular disease prevention, and may, in moderate
amounts, help protect against several postmenopausal ailments. In all these areas, the activity of intestinal microflora is critical, as may be the parallel activity
of other dietary components. Thus, a traditional Japanese diet not only includes
high soy content, but is also low in fat. Therefore, chemopreventive effects of
phytoestrogens suggested by epidemiological evidence may not translate to different races or cultures for which other dietary inputs differ. At present, no definite
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recommendations can be made as to the dietary amounts of phytoestrogens needed
for disease prevention.
One problem for the acceptance of potential health beneficial effects of phytoestrogens is that, to date, no specific mechanism(s) has been proven for their
mode of action. The hormone-dependent cancers, atherosclerosis, and coronary
heart disease are all associated in some way or other with sex hormones and their
metabolism, and phytoestrogens clearly interfere with intracellular steroid hormone metabolism. This may be the key to their activity (3), but direct evidence is
lacking. The other problem is that epidemiological studies yield essentially correlative data, and it is therefore difficult to distinguish between effects of phytoestrogens and other soy or grain fiber components, or indeed other lifestyle factors
that might be associated with a particular type of diet.
A few recent studies on the effects of phytoestrogens on mammalian cells have
utilized DNA arrays to obtain a more global picture of phytoestrogen-mediated
changes in gene expression and to compare these to effects of mammalian estrogens such as estradiol (28, 39, 141). Unfortunately, the precision and resolving
power of this technology is not matched by the overall experimental design. The
problem is that application of pure chemicals to mammalian cells, either directly
or through dietary supplementation, bypasses the important factors of dietary delivery and bioavailability, and dietary supplementation with food sources such as
flaxseed or soy flour adds additional variables (i.e., all the other components of the
supplemented food). It would be extremely helpful for assessing the true effects
of dietary phytoestrogens in animal studies to be able to feed whole food sources
that differ only in the phytoestrogen content. This can now be achieved for the
isoflavones by genetic manipulation of the relevant pathways in transgenic plants
(119, 201) such as Arabidopsis, rice, corn, and alfalfa, all suitable for animal feeding studies. It is expected that similar progress will soon be made with the lignans.
Metabolic and transcriptional profiling of the plant tissue (175, 195) can be used
to confirm that the dietary source of phytoestrogen(s) is not modified in other areas that could impact the experiments. Armed with such source material in which
the phytoestrogens exist as natural glycosides in the correct cellular compartment
within the plant, and in which their nature and levels can be precisely controlled,
it should be possible to use transcriptome profiling in the animal to better understand the molecular impacts of a diet containing phytoestrogens on various target
tissues. Such studies, in concert with more detailed epidemiological and chemical
intervention work, may finally lead to the definition of “recommended daily doses”
for phytoestrogens in relation to specific disease risks.
ACKNOWLEDGMENTS
The author thanks Cuc Ly for assistance with artwork. Work from the author’s
laboratory was supported by the Samuel Roberts Noble Foundation and the Oklahoma Center for the Advancement of Science and Technology Health Sciences
Program.
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The Annual Review of Plant Biology is online at http://plant.annualreviews.org
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Figure 2 (A) Structures of isoflavonoid phytoestrogens and their metabolites, indicating
structural similarities to 17β-estradiol. (B) The biosynthesis of isoflavonoid, flavonoid, and
stilbene phytoestrogens. The enzymes are CHS, chalcone synthase; SS, stilbene synthase;
CHR, chalcone reductase; PT, prenyltransferase; CHI, chalcone isomerase; IFS, isoflavone
synthase (2-hydroxyisoflavanone synthase); IOMT, isoflavone O-methyltransferase;
2HIOMT, 2-hydroxyisoflavanone O-methyltransferase; and DH, 2-hydroxyisoflavanone
dehydratase. The question mark indicates that the dehydratase is not essential because the
reaction can occur nonenzymatically. The different classes of (iso)flavonoids are represented by different colors of shading: orange, stilbene; yellow, chalcone; pink, flavanone;
blue, 2-hydroxyisoflavanone; green, isoflavone.
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Figure 2 (Continued)
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